Actually, to put it the other way round, there are a lot of current things that you can still do on an old Amiga. I'm not sure if that's the issue. Such a project would have a fair audience among people like myself, who would get one just to play with it, admire the design and have it sit near some 20+-year old Amiga.

Legacy Mac platforms are well served by Linux, I have Gentoo on my iBook G3. It has excellent software support, I don't think AmigaOS would bring anything new to such a platform.

Then again -- right now, I don't think there's a platform for it to bring anything new on.